When a workpiece is machined to have a corner shape by regular wire electric discharge machining, a corner shape is first formed by rough machining so that a finishing allowance preliminary remains. Next, the electrical condition for the remaining finishing allowance is gradually changed to that of the final machining, and two or more machining operations are performed with its offset quantity being step by step decreased to improve accuracy in the corner shape (these machining operations are collectively called as skim cuts). The shape accuracy in a corner portion is easily degraded. One of the reasons is that there is a difference between a machining quantity in a straight line portion and a machining quantity in a corner portion. The smaller the corner radius is, the larger the difference between these machine quantities becomes.
In a process of machining a workpiece, machining a corner portion at the same feed speed as that for a straight line portion produces variations in machining quantity to cause an insufficient machining or an over machining. This causes difficulties for machining a corner portion with a high accuracy originally demanded. Therefore, in order to improve corner shape errors produced by insufficient machining or over machining, it is necessary, especially in finish machining, to control to equalize machining quantities per unit time between a straight line portion and a corner portion.
For dealing with such a demand, a controlling method is disclosed in Patent document 1, in which a machining quantity of a corner portion is estimated from a discharge frequency and a voltage drop value in the average machining voltage, to control a machining feed speed at the corner portion according to the discharge frequency and the voltage drop value in the average machining voltage.
In Patent document 2, a machining quantity at a corner portion is approximated by a removal machining distance that is the length of the perpendicular dropped from an intersection point of a discharging gap circle with a previously machined surface to the current machining surface so as to control a machining feed speed at the corner portion according to variations in the removal machining distance.    [Patent document 1] Japanese Patent Application Laid-Open No. 2006-123065    [Patent document 2] Japanese Patent Application Laid-Open No. 2004-148472